The area occupied by the Baths of Diocletian is between the Piazza dei Cinquecento, opposite Termini Station, Piazza della Repubblica, Via Cernaia and Via Volturno, where the main entrance is believed to have been located. At the centre of the opposite side, along the rectangular boundary that delimited the complex, there was a vast exedra, built over in the late 19th century by what is now the Piazza della Repubblica. To the sides of the exedra were two libraries, followed by two circular chambers, one of which was transformed in 1598 into the Church of San Bernardo. The other is still visible at the start of the Via del Viminale. Beyond the entrance, reached through a stately garden, was the central building with the bathing halls. Nothing remains of the caldarium except for a small apse that forms the façade of Santa Maria degli Angeli, while the frigidarium, the imposing hall with its triple-cross vault, and the circular tepidarium can still be seen in the transept and vestibule respectively of the great Basilica. To the sides of the central building, colonnaded gymnasiums were arranged symmetrically, along with a series of numerous other majestic interconnected halls, referred to as the Grandi Aule (Great Halls).
The visit to the museum includes some of the surviving Great Halls. Hall X was one of the monumental entrances to the central building. Inside, current features include the tomb of the Platorini family, discovered in 1880 in the Lungara area on the right bank of the Tiber, and two chamber tombs found in 1951 inside a large core of volcanic tuff along the Via Portuense.
The original function of Hall XI is not known, but it was transformed in the period after its construction into an additional tank to supply water for the swimming pool. A changing room (apodyterium) was found in Room IX. These rooms were used by people frequenting the Baths to leave their belongings before heading towards the gymnasium and commencing the bathing routine.
Room VIII houses some of the large fragments of what remains of the architectural decoration of the Baths. Through a prospect punctuated by pillars and columns, the Hall gave onto the natatio, the monumental swimming pool measuring 4,000 m2 (approximately three times the size of a modern Olympic pool). The 90-metre façade (about twice the size of the corresponding feature at the Caracalla Baths) was constructed using the model of theatrical sets, with ultra-high rectangular and semicircular niches decorated with columns and statues. The surface was entirely covered with coloured marble and mosaics that created spectacular light and colour effects when reflected on the surface of the water. The pool had a depth of 1.30 metres, and the base and walls were covered in slabs of white marble.
The grandeur of the natatio can still be appreciated today despite the loss of the architectural decoration, which was reused over the centuries as construction material for the Charterhouse, and the presence of the small cloister and the presbytery of Santa Maria degli Angeli, which occupy most of the pool structure.
The Octagonal Hall
Probably used as a secondary frigidarium, the Octagonal Hall is square on the outside and octagonal inside, and is surmounted by a majestic ‘umbrella’ dome, once decorated with fine stuccoes. The level of the ancient floor was much lower than its current position. The Hall was in fact transformed in the 17th century into a grain store for the Pontifical Food Administration. It is that era that is the origin of the large masonry pillars that punctuate the underground chamber, where the buildings existing before the construction of the Baths of Diocletian are still visible. In 1928, the Hall was transformed into a Planetarium, through the installation of a projector that reproduced the celestial vault on the interior surface of the dome. The Planetarium was removed in 1987, but its geometric latticework frame on metal columns and cast iron capitals still remains.